1. Field of the Invention
The present invention relates generally to pushbutton switches, and in, particular, to a cover member for an illuminated pushbutton switch having an illumination function for emitting light from the inside to the outside for use with pushbutton switches requiring input operations in a dark place, such as various switches for telephones, AV apparatuses, remote controllers, vehicle interior equipment, and so forth.
2. Description of the Related Art
Illuminated-type pushbutton switches are frequently used for switches requiring input operations in dark places, such as various switches for electronic equipment such as portable telephones and AV apparatuses, and vehicle interior equipment mounted on the vehicle, and so forth. Such illuminated-type switches have a key top with an indicator portion in which a letter, a numeric character, a symbol, a pattern or the like is formed, and through which light from an internal light source such as an LED is emitted. This type of switch is designed to enable an operator to easily recognize such an indication symbol and to provide input for an apparatus with reliability during the night or in dark places.
FIGS. 7A, 7B, and 8 show an example of a conventional illuminated pushbutton switch. A cover member 1 for this illuminated pushbutton switch is an example of a four-direction key designed to enable independent input operations at different positions. The structure of this illuminated pushbutton switch cover member 1 will be described. The cover member 1 is constructed by a solid resin key top 2 made of a transparent hard resin, such as polypropylene. The key top 2 is bonded and fixed to a key sheet 3 formed of a transparent silicone rubber. On the resin key top 2 are formed a transparent layer 4, a light-blocking indication layer 5 in which indicator portions 5b are formed as triangular symbols 5a in a blanking manner for indication of four directions, and a colorless transparent protective layer 6. The resin key top 2 is bonded and fixed to a key top supporting portion 3a of the key sheet 3. The key top supporting portion 3a has pressing projections 3bformed just below the triangular symbols 5a. A contact 3c formed from an electroconductive ink or the like is provided on an end of each pressing projection 3b. 
To perform a switch function, the contact 3c is brought into contact with a contact Pc on a printed circuit board P (hereinafter referred to as (“circuit board”) mounted in a case C of an apparatus. The key sheet 3 has a flexible skirt portion 3d extending from the entire periphery of the key top supporting portion 3a obliquely downwardly so as to broaden toward the end. When an operator presses the resin key top 2, this thin skirt portion 3d is flexed so that the operator feels a click. The key sheet 3 has a base portion 3e formed at the lower peripheral end of the skirt portion 3d. The base portion 3e is placed on the circuit board P.
Referring to FIG. 8, in the illuminated pushbutton switch cover member 1 constructed as described above, light emitted from a light source PL provided on the circuit board P passes through the transparent key sheet 3 and the resin key top 2, is diffused by the transparent layer 4, and passes through the transparent protective layer 6 while internally illuminating the triangular symbols 5a formed in the indicator portions 5b. This is the illumination mechanism of the conventional pushbutton switch cover member 1.
The above-described conventional illuminated pushbutton switch cover member 1 has a certain effect of illumination of the triangular symbols 5a formed in the indicator portions 5b, but has a problem in that the intensity of light for illumination is low. That is, in this illuminated pushbutton switch cover member 1, the transparency of each of the key sheet 3 and the resin key top 2 is set substantially high in order that each triangular symbol 5a be illuminated as a clear indication at a high luminance. In this illuminated pushbutton switch cover member 1, however, a considerable amount of light leaks to the interior of the case C through the transparent key sheet 3 and the resin key top 2 (see FIG. 8). Therefore, light is also emitted to the outside through a gap Cd between a case surface defining a window hole Ch formed in the case C and the peripheral surface of the resin key top 2 (indication layer 5). This emission of light lowers the contrast of the illumination in the triangular symbols 5a with the surrounding portion to reduce the apparent sharpness of the indication.
As a method for solving this problem, a method of improving the combination of the pushbutton switch cover member 1 and the circuit board P, for example, by changing the position of the light source PL from the circuit board P to a place in the pushbutton switch cover member 1, and a method of improving only the pushbutton switch cover member 1, are conceivable. In view of the manufacturing cost and the like, the method of improving only the pushbutton switch cover member 1 for solution of the problem is preferred. For example, as a method of improving only the pushbutton switch cover member 1 for solution of the problem, a method of coloring the key sheet 3 or a method of additionally forming a light-blocking layer on the skirt portion 3d of the key sheet 3 to prevent transmission of light through the corresponding portion of the key sheet 3 may be considered.
The method of coloring the key sheet 3, however, requires a secondary vulcanization process after kneading and forming silicone rubber, i.e., the material of the key sheet 3, and therefore entails the drawback of increasing the manufacturing cost. This method also entails a problem in that a low-molecular-weight siloxane component remaining in silicone rubber is scattered to cause a continuity fault or the like in the switch. On the other hand, the method of additionally forming a light-blocking layer has the drawback of increasing the number of process steps and is disadvantageous in terms of manufacturing time and cost.
While the state of the related conventional art is as described above, the miniaturization of illuminated pushbutton switches and the diversification of illuminated pushbutton switches accompanying the development of apparatuses having an increased number of functions have been advanced to meet market needs. Therefore there is a demand for further meticulous designs. If indicator portions including display of characters, numeric characters, symbols, and so forth, formed in an indication layer are designed more meticulously, and if a need arises to enable visual recognition of details of such indicator portions, it is necessary to increase the intensity of light for illumination in order to improve the visibility of details of such indicator portions, and it is also necessary to realize higher illumination at a low cost.